The present invention relates to a conductive sintered compact for fixing electrodes in an electronic device envelope. More particularly, the present invention relates to a conductive fixing member for arranging electrodes mounted on predetermined areas in an electronic device envelope. For example, the present invention relates to a conductive sintered compact used to fix grid electrodes on the surface of an insulating substrate disposed inside a fluorescent display tube envelope.
Conventionally, conductive fixing members acting as internal wiring layers are used inside an envelope constituting a fluorescent display tube or the like to arrange various electrodes over the insulating substrate therein.
Usually, in order to prepare a conductive paste, Ag particles, acting as a conductive material, and a low melting point glass, acting as a fixing component, are first immersed and kneaded in an organic vehicle. The resultant conductive paste is applied over an insulating substrate by a screen printing process and then formed in a predetermined pattern. At this time, electrodes to be fixed are placed over the patterned conductive compound (the conductive paste). By baking the insulating substrate in a high temperature atmosphere, the conductive compound is solidified to fix the electrodes firmly. That is, the conductive sintered material containing Ag particles and low melting point glass as main components, or the conductive member for fixing electrodes, is obtained.
Particularly, as to fluorescent display tubes, the grid electrodes are firmly fixed and arranged over a glass substrate (an insulating substrate) with the conductive compound. Since the conductive compound is baked and solidified at a relatively low temperature of 200° C. to 500° C., lead series glass is used as a main fixing component. In this case, the conductive sintered material is called a grid intermediately-bonded electrode. This technique, called a grid intermediately-bonding system, has been widely utilized conventionally.
FIG. 1 shows the main structure of a conventional general fluorescent display tube utilizing the grid intermediately-bonding system. The fluorescent display tube shown in FIG. 1 can be conceptually applied to the grid intermediately-bonding system (to be described later in detail) utilizing the conductive sintered material according to the present invention.
In the configuration shown in FIG. 1, the fluorescent display tube includes a glass substrate 1 having an upper surface on which a SiO2 film 11 is formed. An anode wiring layer 2a and a grid wiring layer 2b are formed on the SiO2 film 11, respectively. An insulating layer 8 is formed over the wiring layers 2a and 2b. A grid electrode 9 in a predetermined shape on a predetermined area of the insulating layer 8 and confronting the cathode electrode 10 is firmly fixed and anchored with the intermediately-bonded electrode (a conductive sintered material) 6.
An anode electrode 5, formed of an anode conductor 3 and a fluorescent substance 4, is formed over the one anode wiring layer 2a via the through hole 7 opened in the insulating layer 8. The electrode 6 is formed over the other grid wiring layer 2b via the through hole 7 opened in the insulating layer 8, to securely fix the grid electrode 9.
Referring to FIG. 1, numeral 12 represents a crystalline glass formed over the electrode 6; numeral 13 represents a front plate; and numeral 14 represents a side plate. These elements form an envelope. Numeral 15 represents a metal lead taken out from the terminal electrode 16.
In the general grid intermediately-bonding system (that is, the conventional system), a conductive material, such as Ag particles, and a low melting point glass being a fixing component are used as main components for the electrode 6. First, the main components are immersed and kneaded in an organic vehicle to form a conductive paste having a viscosity needed as an adhesive agent. The resultant conductive paste is coated over the insulating substrate by the screen printing process and is shaped in a desired pattern. Meanwhile, the grid electrode 9 is disposed at a predetermined position in the predetermined area.
Next, the insulating substrate, on which the grid 9 is formed over a conductive paste pattern, is calcined in an atmosphere of 300° C. to 500° C. During this baking, the organic vehicle contained in the conductive paste is evaporated and sputtered, and the low melting point glass is crystallized. The resulting conductive sintered compact (or body) is used as an electrode.
In more detail, when the electrode 6 is used, the leg of the grid electrode 9 is attached at the predetermined position of the glass substrate 1, utilizing the viscosity of the conductive paste. The glass substrate 1, on which the grid electrode 9 is attached with the conductive paste pattern, is calcined. In this baking process, the low melting point glass in the paste is once fused and solidified. Thus, the grid electrode 9 is securely fixed to the glass substrate 1, as desired. The conductive material of the electrode 6 to be solidified electrically connects the grid 9 to the grid wiring layer 2b. 
In that case, a conductive paste acting as the electrode 6 is prepared. The conductive paste, for example, is prepared by immersing and kneading a conductive material made of 36.7 wt % of Ag particles and 20 wt % of Al particles, a low melting point glass, lead titanate (43.3 wt %), and a metal oxide series pigment, in an organic vehicle. The resultant conductive paste is coated using a screen printing process and is shaped in a predetermined electrode pattern.
Next, the grid electrode 9 is held at a predetermined position with the conductive paste. Then, the intermediate structure is calcined at a temperature of 300° C. to 500° C. and then cooled down to obtain the electrode 6. The grid electrode 9 is securely anchored on the glass substrate 1 with the welded electrode 6. This can reduce the cracking, which occurs in the glass substrate. This technique is disclosed in Japanese Patent Laid-open Publication No. Tokkai-hei No. 3-152837.
Moreover, Japanese Patent Laid-open Publication No. Tokkai-hei 4-269404 discloses a similar technique of reducing the generation of cracks in a glass substrate. According to this art, 40 wt % of a conductive material made of a mixture of Ag particles and 10 wt % to 100 wt % of graphite particles, a Pb—Si—Zn—B series low melting point glass, and an organic vehicle are used for the conductive paste.
Moreover, Japanese Patent Laid-open Publication No. Tokkai-hei 7-254360 discloses the technique of reducing the cracking and flaking of the electrode 6 itself. According to this art, a conductive material made of 36.5 wt % to 50 wt % of Ag particles, a low melting point glass, 39 wt % to 50 wt % of lead titanate acting as a filler, 2 wt % to 18 wt % of an organic metal, and an organic vehicle are used for the conductive paste. The grid electrode 9 is fixed with the conductive paste. Then, the intermediate structure is baked at 300° C. to 500° C. and then cooled to make the electrode 6.
However, as described in the publication No. 7-254360, the organic vehicle in the conductive paste is dissolved and evaporated at about 180° C. during the heat treatment process. Thus, the conductive paste itself is dried and loses its viscosity. The softening temperature of the fritted glass formed of a low melting point glass is 320° C. Therefore, the adhesive force of the conductive composition paste at the interface between the glass substrate 1 and the grid electrode 9 is significantly reduced during the heat treatment in the temperature range.
The glass substrate 1 and the grid substrate 9, each having a different thermal expansion coefficient, expand thermally in the heat treatment so that a difference occurs between the thermal expansion amounts. For that reason, an internal stress exceeding a largely-reduced adhesive force of the conductive composition paste occurs at the interface between the glass substrate 1 and the grid electrode 9 bonded with the electrode 6. As a result, cracking can occur between the electrode 6 and the grid electrode 9 and the glass substrate 1, so that they may be finally peeled off from the glass electrode 1.
In order to overcome peeling caused by the cracking of the electrode 6, the electrode 6 is formed using the following conductive compound paste, as described previously. That is, the conductive compound paste is made of a conductive material made of 36.5 wt % to 50 wt % of Ag particles, a low melting point glass, 39 wt % to 50 wt % of lead titanate acting as a filler, 2 wt % to 18 wt % of an organic metal, and an organic vehicle.
The fixing strength was confirmed by peeling off the grid electrode 9 bonded to the electrode 6, with a spring balance. By peeling off the grid electrode 9, the electrode 6 fixed to the grid electrode 9 was peeled off from the substrate surface.
The soda lime glass has a thermal expansion coefficient of 85 to 90×10−7/° C. The insulating layer has a thermal expansion coefficient of 65 to 80×10−7/° C. The grid material formed of an alloy (of 42% Ni-6% Cr-residual Fe), SUS430 alloy, SUS398 alloy, and SUS343 alloy has a thermal expansion coefficient of 100×10−7/° C. (at 400° C.). According to one theory, differences in thermal expansion coefficient between the interfaces of these substances produce an internal distortion. Finally, the internal distortion cracks the electrode 6.
Moreover, a conductive paste not containing lead, which is an environmental load matter, for wiring conductors formed on an insulating substrate is disclosed in Japanese Patent Laid-open Publication No. Tokkai-hei 11-329072 and No. Tokkai 2000-48642. In this case, the thickness of the conductive sintered compact made of the conductive paste for wiring is 10 μm or less.